This invention relates to the control of adhesive flow through a nozzle. More particularly, this invention relates to controlling adhesive flow through a nozzle such that a consistent, continuously controlled volume of adhesive is dispensed on a moving substrate independent of other system and environmental parameters.
In the tobacco industry, smoking articles, such as cigarettes, are made by cut filler being air-formed into a continuous rod of tobacco on a travelling belt. The tobacco rod is then encircled by a continuous ribbon of paper which is glued and heat-sealed to form a side seam. The formed rod proceeds to a cutting machine or further processing machine. Within this rod making process, formation of a consistent high-quality side seam requires a consistent and continuous amount of glue or adhesive dispensed for heat-sealing. Cost-effective high speed cigarette production also demands controlling the amount of glue or adhesive to maintain quality control.
However, currently employed adhesive dispensing methods do not meet the rigorous standards of high speed production because they are dependant on external conditions. External conditions affecting the dispensing of adhesive include temperature changes, pressure changes, viscosity and dispensing nozzle wear. One common adhesive dispensing method is the use of head, i.e., the pressure of the adhesive as a fluid created by maintaining the adhesive in a reservoir elevated above the dispensing nozzle. This method is unsatisfactory because as the adhesive level in the reservoir goes down, the head goes down and so does the flow rate. As a result, some portions of the tobacco rod have more glue or adhesive than other portions of the same tobacco rod.
Adhesive dispensing is also controlled by maintaining constant adhesive pressure behind the dispensing nozzle. For example, U.S. Pat. No. 2,914,253 to Jobus et al. maintains nozzle back pressure through use of a recirculation loop. U.S. Pat. No. 4,842,162 to Merkel discloses an adhesive dispensing nozzle controlled in part by a flow sensor located at the nozzle. However, the flow rate sensor is in fact a pressure sensor located downstream of the needle control valve. See also U.S. Pat. No. 4,613,059 to Merkel.
In another alternative, U.S. Pat. No. 4,858,172 to Stern teaches use of a positive displacement pump to guarantee a constant flow rate of adhesive to a sealant gun. Stern also discloses a closed loop flow control using feedback from a flow meter used in place of the positive displacement pump to provide accurate steady state performance. But this disclosure is conditioned by an earlier statement that even placing a flow meter very near the output nozzle and forming a closed loop flow control does not satisfy control of flow independent from outside conditions such as temperature change and nozzle wear. U.S. Pat. No. 4,711,379 to Price teaches a related method of variable proportional flow of adhesive relative to a substrate speed using a torquemotor.
Also, U.S. Pat. No. 4,662,540 to Schroter teaches dispensing a high viscosity liquid using a flow detector to ensure constant flow. However, there the flow detector does not control the flow. Instead, a pressure detection system controls the flow.
While each of the above references teaches control of adhesive flow through various methods, none of them teach use of a flow meter which is not dependent on outside parameters such as temperature, pressure, viscosity or nozzle wear. This dependence presents a drawback in each prior method that is overcome by the present invention. Furthermore, the present invention provides a consistent controlled volume of adhesive at low flow rates, which is not contemplated by the cited disclosures.